Liquefied natural gas (LNG) is principally liquid methane, with smaller amounts of C2+ hydrocarbons also present. It is prepared by chilling a raw natural gas stream to a temperature and at a pressure to cause at least a portion of the methane in the raw gas to condense as a liquid. The natural gas stream used to prepare LNG may be recovered from any process which generates light hydrocarbon gases. Generally, the raw natural gas from which LNG is prepared is recovered from a crude oil or gas well.
Raw natural gas, in addition to the presence of methane, typically will also include varying amounts of C2− hydrocarbons; C3 hydrocarbons; and C4 hydrocarbons. Natural gas which also comprises varying amounts of C5+ hydrocarbons is referred to as “wet natural gas” while “dry natural gas” comprises little or no C5+ hydrocarbons. As used herein, C1 represents a hydrocarbonaceous compound having one carbon atom per molecule; C2 has two carbon atoms per molecule, etc. C3-C4 represents a hydrocarbonaceous material, comprising predominately compounds having three carbon atoms per molecule and/or compounds having four carbon atoms per molecule. C5+ represents compounds having five or more carbon atoms per molecule. Methane is a representative example of a C1 compound and is the principal constituent of raw natural gas. Ethane, ethylene, and mixtures thereof are representative examples of a C2 compound. Propane, propene, butane, butenes and mixtures thereof are representative examples of a C3-C4 compound. Pentanes, isobutane, pentenes, hexanes, hexenes and comparable higher molecular weight species, and their mixtures, are representative of C5+ compounds.
The process of liquefying natural gas involves chilling the raw natural gas, either at atmospheric or super-atmospheric pressure, until the methane and ethane condense as liquids (LNG). On account of their higher molecular weights, any C3+ vapors contained in the raw natural gas will condense prior to the condensation of the C1 and C2 compounds, forming a liquid product termed “natural gas liquids”. Each of the components which condense during the preparation of LNG has important commercial value. As already noted, C1 and C2 compounds are the major components of LNG. Any heavier materials which are present in the raw natural gas are carefully removed prior to condensing the LNG. Liquefied petroleum gas (LPG), comprising C3-C4 hydrocarbons, is important as a refrigerant in the chilling process. LPG is also useful as a fuel in the LNG liquefaction process and has value as a transportation fuel. The C5+ condensate recovered from the raw natural gas is valuable as a blending component for fuels, particularly for transportation fuels. It is therefore important that the liquefied C5+ condensate and the C3-C4 LPG be prepared separately from the LNG. Where propane and/or butane are important products, they are stored in separate storage vessels as relatively pure hydrocarbons.
LPG (i.e., propane and butane) is typically stored in tanks at atmospheric or super-atmospheric pressure. The choice is primarily one of economics and compatibility with associated processes and equipment. LPG stored in tanks at atmospheric pressure is maintained at low temperatures (−40° F. for the propane and 0° F. for the butane) to maintain the material as a liquid. Heat absorbed into the tank from the surrounding ambient conditions cause both the propane and the butane to continuously boil off some amount of vapor, producing boil off gas (BOG). Typically, the propane and butane vapors are recovered by compressing the vapors with a screw or a reciprocating compressor from less than about 1 psig to about 200 psig and about 50 psig respectively, to reach the appropriate pressure-temperature equilibrium point (˜100° F.) to allow a cooling water exchanger or a fin fan to provide sufficient heat removal to condense the vapors. Since propane and butane each condense at a different temperature, each stream requires a separate compressor, knockout drum, condensing exchanger and cooling medium. Furthermore, the propane and butane streams cannot be combined into one recovery stream as the combined stream will contaminate the pure component tank. The recovery systems also require some back-up power generation system to drive the compressors in the event of a power failure, since pressure cannot be allowed to build in the tank or vapors to be vented to atmosphere.
LNG storage tanks have a boil off gas (BOG) recovery system including a blower and a recovery line from the storage tanks to either a flare or a location in the LNG process that can recover the low pressure LNG vapor stream (blowers are typically used when a fairly low increase in pressure is required). See for example, U.S. Pat. No. 6,470,706.
The present invention is directed to an efficient process for preparing and storing separate LPG streams in the process of preparing LNG.
As used in this disclosure the word “comprises” or “comprising” is intended as an open-ended transition meaning the inclusion of the named elements, but not necessarily excluding other unnamed elements. The phrase “consists essentially of” or “consisting essentially of” is intended to mean the exclusion of other elements of any essential significance to the composition. The phrase “consisting of” or “consists of” is intended as a transition meaning the exclusion of all but the recited elements with the exception of only minor traces of impurities.